Composite color-plus-clear coatings are widely utilized in the coatings art. They are particularly desirable where exceptional gloss, depth of color, distinctness of image, or special metallic effects are required. The automotive industry has made extensive use of color-plus-clear composite coatings for automotive body panels.
As used herein, the term "composite color-plus-clear" relates to composite coating systems requiring the application of a first coating, typically a colored basecoat coating, followed by the application of a second coating, generally a clearcoat, over the noncured or "wet" first coating. The applied first and second coatings are then cured. Thus, such systems are often described as "wet on wet" or "two coat/one bake". Drying processes which fall short of complete cure may be used between the application of the coatings.
Clearcoats used in color-plus-clear systems must have an extremely high degree of clarity in order to achieve the desired visual effects. High gloss coatings also require a low degree of visual aberrations at the surface in order to achieve the desired visual effect such as high distinctness of image (DOI). As a result, clearcoats of color-plus-clear systems are especially susceptible to the phenomenon known as environmental etch, i.e., spots or marks on or in the clear finish that often cannot be rubbed out.
Although many compositions have been proposed for use as the clearcoat of a color-plus-clear composite coating, prior art clearcoat coatings often suffer from disadvantages such as coatability problems, compatibility problems with the colored basecoat, solubility problems, and/or insufficient resistance to environmental etch.
Clearcoat coating compositions exhibiting many advantages over prior art compositions, especially with respect to environmental etch, were disclosed in U.S. Pat. Nos. 5,474,811 and 5,356,669. The disclosed clear coating compositions comprise a first component comprising a polymer backbone having appended thereto at least one carbamate functional group, and a second component comprising a compound having a plurality of functional groups that are reactive with said carbamate groups.
However, regardless of the foregoing improvements, the application of a clear coating composition over a colored basecoat in a wet-on-wet or two coat/one bake system may sometimes result in an undesirable phenomenon known as "wrinkling".
Wrinkling often manifests as one or more small furrows or ridges in the cured clearcoat. Most often, there appears a plurality of furrows, often in areas of low clearcoat film build and high basecoat film build. It is theorized that such imperfections are caused by folding or "wrinkling" of the clearcoat during the curing process.
While not wishing to be bound to a particular theory, it is believed that wrinkling may occur as a result of the presence of or reaction between volatilized amines and acid catalysts. Such a combination can result from the use of a basecoat composition employing waterborne technology, especially anionic stabilized waterborne systems requiring the use of volatile amines, and a solvent borne clearcoat composition utilizing strong acid catalyzed melamine formaldehyde crosslinkers.
Cured color-plus-clear composite coatings exhibiting wrinkling do not possess the required appearance and/or degree of clarity. Composite coatings vulnerable to wrinkling are viewed unfavorably by the automotive industry.
Thus, there is a need for a curable coating composition suitable for use in a color-plus-clear composite coating which possesses desirable curing, performance, and appearance properties but which does not exhibit wrinkling upon cure of the composite coating. Desirable performance properties are good solvent resistance, hardess and environmental etch resistance. The coating should also cure at commercially acceptable curing conditions. Advantageous appearance properties are a color resulting solely from the desired pigmentation, high DOI, and a glossy appearance over a wide range of basecoat and clearcoat film thickness.
In particular, it would be advantageous to provide curable coating compositions comprising carbamate functional resins which, when used in a color-plus-clear composite coating, do not exhibit wrinkling upon cure of the composite coating.
Most particularly, it would be desirable to provide clearcoat coating compositions which, when used as the clear in a color-plus-clear composite coating, provide composite coatings having advantageous performance and environmental etch properties but which are substantially free of wrinkling.
Finally, it would be desirable to provide a method for providing substantially wrinkle free color-plus-clear composite systems.